Prevent unnecessary $wpdb->update() when hierarchical post doesn't have children

[danielbachhuber]

Includes tests for reassignment behavior when deleting a hierarchical post, which I couldn't find in our test suite.

[joehoyle]

Looks good, as noted in Slack, the count I think is superfluous, checking the result from get_results is falsy should be fine.

[danielbachhuber]

Updated patch to remove count() call

[joehoyle]

In 35817:

Prevent unnecessary $wpdb->update() when hierarchical post doesn't have children.

Props danielbachhuber.
Fixes #34896.